1. Field of the Invention
The invention relates to a biaxial wheel test assembly for fatigue and durability testing of a wheel-and-tire assembly. More particularly, the invention relates to an improved biaxial wheel test assembly which measures actual loads experienced by a wheel-and-tire assembly and automatically controls a camber angle of the wheel-and-tire assembly in response to monitoring a camber moment during testing.
2. Description of Related Art
Biaxial wheel testing apparatuses are well known in the art for fatigue and durability testing of vehicle wheel components including wheel rims, wheel hubs, wheel bearings, wheel bolts, and/or other braking, steering, and suspension components. Such testing apparatuses subject passenger car and light truck wheels to simulated real-world road conditions by mounting a wheel-and-tire assembly to a spindle, disposing the wheel-and-tire assembly into a rotating drum, and subjecting the wheel-and-tire assembly to various predetermined loads. The wheel-and-tire assembly includes a tire mounted to a wheel rim. A portion of the tire, or tire contact patch, engages an inner circumferential surface of the rotating drum.
U.S. Pat. No. 4,475,383 issued to Fischer et al. discloses one example of a biaxial wheel testing apparatus which includes a loading device or frame for imposing selected vertical or radial input loads that are directed radially toward an axis of rotation of a wheel-and-tire assembly, and lateral or axial input loads that are directed parallel to the axis of rotation of the wheel-and-tire assembly. The radial input loads are imposed on the wheel-and-tire assembly by a first servo-controlled hydraulic cylinder acting upon the frame to force a tire of the wheel-and-tire assembly against an inner surface of a cylindrical drum. Similarly, the axial input loads are imposed on the wheel-and-tire assembly by a second servo-controlled hydraulic cylinder acting upon the frame to force the tire against an annular contact ring fixedly attached to the inner surface of the drum.
U.S. Pat. No. 7,254,995 issued to Leska, Sr. et al. discloses another example of a biaxial wheel testing apparatus which includes a support structure that supports a wheel-and-tire assembly in engagement with a rotating drum. A slide assembly, connected to a first hydraulic actuator, is provided for movement of the rotating drum substantially parallel to an axis of rotation of the rotating drum to apply lateral or axial input loads to the wheel-and-tire assembly. The wheel-and-tire assembly is mounted to a spindle which, in turn, is movably supported by a plurality of struts operatively connected to a base. A first strut is operatively connected to a second hydraulic actuator for applying a radial input load through the first strut so as to simulate substantially vertical loads to the wheel-and-tire assembly. The radial input loads are measured by a first load cell disposed along the first strut. The axial input loads are reacted through a second strut connected between the spindle and the base. The axial input loads are measured by a second load cell disposed along the second strut. A pair of third struts support the spindle in a vertical direction and each third strut includes a third load cell disposed therealong for measuring drive torque and braking torque. A camber angle of the wheel-and-tire assembly can also be adjusted by rotating the base using a third hydraulic actuator.
One particular disadvantage of the biaxial wheel testing apparatus disclosed in Leska, Sr. et al. is the load cells are located remotely from the wheel-and-tire assembly and measure the input loads as opposed to the actual loads experienced by the wheel-and-tire assembly itself. Therefore, complex mathematical algorithms are necessary to estimate the actual loads experienced by the wheel-and-tire assembly. Another disadvantage is frictional and hysteresis losses occurring within the support structure between the load cells and the wheel-and-tire assembly that are difficult or impossible to account for. Such algorithms and losses can introduce errors in the test data and provide failure modes that are unrealistic relative to that observed during real-world vehicle testing or provide erroneous or non-repetitive wheel damage.
Additionally, the above-described biaxial wheel testing apparatuses are relatively large and bulky in order to provide the necessary movement of the wheel-and-tire assembly. Further, hydraulic systems used to load the wheel-and-tire assembly against the rotating drum have inherent disadvantages in that supply and return lines must be routed for each hydraulic actuator. With multiple hydraulic line connections, leaks are unavoidable.
In use, the above-described biaxial wheel testing apparatuses test wheel-and-tire assemblies using sets of lateral and radial input loads determined during vehicle testing. Each set of lateral and radial input loads is referred to as a load pair. For each load pair, an optimum tilt or camber angle is determined via correlation to vehicle testing or one-time measurements on other test machines. Once the optimum camber angle is established for a given load pair, this angle is fixed and is used for any subsequent testing using the biaxial wheel testing apparatus. Thus, when a particular wheel-and-tire assembly is subjected to a particular load pair during testing, the predetermined optimum camber angle is selected and fixed until the desired number of revolutions of the wheel-and-tire assembly has been completed. Using the predetermined and fixed optimum camber angle does not compensate for real-time factors that influence the effective loading of the wheel-and-tire assembly and hence the damage experienced by the wheel-and-tire assembly such as: tire-to-drum friction; friction variation with temperature at the tire contact patch; wheel-and-tire geometry, size and stiffness; tire rubber formulation; tread design and wear; and tire debris within the rotating drum. These factors, as well as others, ultimately influence the position of the tire on the drum and its relative interaction therewith. The main effect of this fixed, optimum tilt or camber angle method is a random and unquantifiable source of variation in the test results and an inability to test with significant statistical correlation using different biaxial wheel testing apparatuses.
It is desirable, therefore, to provide an improved, compact, biaxial wheel test assembly that accurately measures loads experienced by a wheel-and-tire assembly and provides direct feedback to a control and data collection system. It is also desirable to provide a biaxial wheel test assembly that automatically and in real-time compensates for factors that influence the effective loading of a wheel-and-tire assembly and hence the damage experienced by the wheel-and-tire assembly.